It has been recognized in connection with cooling devices that energy consumption and adverse temperature fluctuations within a refrigerated space can be minimized by periodically operating a defrost heater to defrost the coils of an evaporator when a predetermined frost load has accumulated thereon. In general, it is desirable to defrost as infrequently as possible, but it is not desirable to allow very large frost loads to develop because they require more time and electrical energy to remove, thus reducing the operating efficiency of the cooling device.
The cooling device is thus operated in two modes of operation, i.e. a cooling mode in which the cooling apparatus of the device is periodically turned on and off in dependence upon a user-adjustable set point and a defrost mode in which the evaporator coils of the cooling device are heated by the defrost heater to remove a frost load therefrom.
Various systems have been devised which calculate or otherwise determine when a defrost mode of operation should be initiated and terminated. For example, some systems accumulate compressor run time and initiate a defrosting operation when a predetermined amount of compressor run time has occurred since the last defrost operation. Other systems, for example as disclosed in Clarke et al U.S. Pat. No. 4,327,557, assigned to the assignee of the instant application, initiate a defrost operation at a time which is determined by taking into account various types of information, such as the number and duration of freezer and fresh food compartment door openings, the duration of the previous defrosting operation and the total accumulated compressor run time since the previous defrost operation.
Still further types of refrigerator controls include a latch circuit which initiates the defrost and cooling modes. A device, such as a frost sensor or timer, initiates the defrost operation by setting the latch circuit, which causes a defrost heater to be energized. A temperature sensor responsive to the evaporator reaching a predetermined elevated temperature resets the latch circuit to terminate defrost and begin a cooling mode by applying power to a compressor control circuit. During the cooling mode, the compressor cycling is controlled by the evaporator temperature sensor or an additional temperature sensor disposed within the refrigerated compartment. The temperature sensors, which may be thermistors, are each coupled to a transistor, the outputs of which are connected together in a logical sum configuration to in turn control the compressor. Such a control is disclosed in Sakamoto U.S. Pat. No. 3,553,975.
The above-noted Sakamoto patent also discloses that the compartment temperature sensor can be inhibited or prevented from operating the compressor when in the defrost mode by the use of a sensing line that changes state with the operational mode. Also disclosed is a sensing line for disabling a defrost terminate temperature sensor when the associated evaporator is not being defrosted which includes a diode that charges a capacitor to alter the voltage bias of the temperature sensor in the presence of an AC voltage on the sensing line.
Mitani et al U.S. Pat. No. 4,294,081 discloses a refrigerator which includes two independent temperature sensing circuits each comprising a temperature sensor, a pair of voltage comparators and a latch. One temperature sensing circuit operates the compressor when compartment temperature rises above a predetermined value and also terminates a defrost operation. The other sensing circuit energizes a defrost heater when frost builds up into contact with the sensor, provided the other sensing circuit is not energizing the compressor.
Wakai et al U.S. Pat. No. 3,483,919 discloses a refrigerator having a freezer thermistor coupled in series with one of a pair of bias resistors through a switch which is operated in dependence upon whether a cooling or freezing mode is selected. When the refrigerator is operated in the cooling mode, the switch couples one of the bias resistors to the freezer thermistor, in turn inhibiting control of the cooling apparatus by the freezer thermistor.
A still further type of defrost control is disclosed in Janke et al U.S. Pat. No. 4,530,218, issued July 23, 1985, entitled "Refrigeration Apparatus Defrost Control" and assigned to the assignee of the instant application. This control system utilizes a temperature responsive switch for switching the apparatus between defrost and cooling modes of operation. The temperature responsive switch is in turn controlled by a heater which is energized upon termination of a defrost cycle to positively effect transfer of the switch to a position which initiates the cooling mode of operation.
Other types of refrigerator controls have been devised for operating a refrigerator in low ambient temperatures. For example, Tershak et al U.S. patent application Ser. No. 588,304, filed Mar. 12, 1984, entitled "Apparatus and Method for Controlling a Refrigerator in Low Ambient Conditions", and assigned to the assignee of the instant application discloses a refrigerator control which inhibits operation of the cooling apparatus in the refrigerator for a period of time when an abnormal condition is sensed resulting from operation in low ambient temperatures.